Rating of SCR

Ratings of SCR

Content

• Rating of SCR

• Voltage Rating

• Current Rating

• Temperature Rating

• Power Rating

Rating of SCR

• The reliable operation of the SCR can be ensured only if it is operated such that its ratings are not exceeded.

• Each thyristor or SCR is manufactured to a particular current, voltage, power, temperature and switching frequency limits within which they can operate reliably.

Types of Rating

These ratings can be

• Continuous,

• Non-repetitive or surge and

• Repetitive ratings.

Surge and repetitive ratings are corresponding to peak values of the SCR.

Nomenclature

The first subscript indicates the state of the SCR and includes

• F- Forward bias

• R- Reverse bias

• T- ON state

• D- Forward blocking state with gate open

The second subscript indicates the operating values

• T- Trigger

• S- Surge or Non-repetitive value

• R- Repetitive value

• W- Working value

Voltage Ratings of SCR

• Peak Working Forward-blocking Voltage V^DWM

• Peak Repetitive Forward-blocking Voltage V^DRM

• Peak Non-repetitive or Surge Forward-blocking Voltage V^DSM

• Peak Working Reverse Voltage V^RWM

• Peak Repetitive Reverse Voltage V^RRM

• Peak Non-repetitive or Surge Reverse Voltage V^RSM

• ON-state Voltage V^T

• Gate Triggering Voltage V^GT

• Forward dv/dt Rating

• Voltage Safety Factor V^f

Peak Working Forward-blocking Voltage V

• It specifies the maximum instantaneous value of forward blocking voltage across the SCR

excluding all surge and repetitive transient voltages.

• Beyond this value of the voltage the SCR cannot withstand during its operation.

Peak Repetitive Forward-blocking Voltage V

• It is the maximum transient voltage that the SCR can block during it’s the forward blocking state repeatedly or periodically.

• This voltage V_DRM is encountered or appeared across the SCR , when the SCR is turned OFF or commutated or due to diodes in the

converter circuit.

Peak Non-repetitive or Surge Forward-blocking Voltage V

• This is the maximum instantaneous value of forward surge voltage across the SCR that is of non-repetitive.

• This V_DSM is less than the forward break over voltage V_BO and this value is in the range

about 130 percent of V_DRM.

Peak Working Reverse Voltage V

• This is the maximum instantaneous value of reverse voltage across the SCR excluding all surge and repetitive transient voltages.

• This V_RWM is equal to the maximum negative value of the supply voltage wave shown in figure.

Peak Non-repetitive or Surge Reverse Voltage V

• It refers to the maximum value of reverse

transient voltage across the SCR that is of non- repetitive.

• This V_RSM is less than the reverse break over

voltage V_BR and this value is in the range about 130 percent of V_RRM.

ON-state Voltage V

• This is the voltage drop between the anode and cathode with specified junction

temperature and ON-state forward current.

• Generally, this value is in the order of 1 to 1.5 Volts.

Gate Triggering Voltage V

• This is the minimum voltage required by the gate to produce the gate trigger current.

Forward dv/dt Rating

• This is the maximum rate of rise of anode voltage that will not trigger the SCR without any gate pulse or

signal.

• If this value is more than the specified value, the SCR may be switched ON.

• This type of triggering is called as false triggering and in practice it is not employed.

• This rating depends on the junction temperature. If the junction temperature is high, the dv/dt rating of the

SCR is lower and vice-versa.

• With the use of snubber networks across the SCRs, it is possible to limit the maximum dv/dt applied to the

SCR.

Voltage Safety Factor V

• Generally, the operating voltage of the SCR is kept below the V_RSM to avoid the damage to the SCR due to uncertain conditions.

Therefore, the voltage safety factor relates the operating voltage and V_RSM and is given as

Current Rating of SRC

• Average ON-state Current Rating I^TAV

• RMS ON-state Current I^TRMS

• Surge Current Rating I^TSM

• I2t Rating

• di/dt Rating

• Latching Current I^L

• Holding Current I^H

• Gate Current I^G

Current Ratings of SCR

• Basically an SCR is a unilateral device and hence average current rating is assigned to it (while RMS current rating is assigned to bilateral devices).

• An SCR has low thermal capacity and short time constant. This means the junction temperature exceeds its rated value even for short over

current.

• This may lead to damage the SCR.

• Current ratings must be properly selected for long life of SCR ,

Average ON-state Current Rating I

• This is the maximum repetitive average value of forward current that can flow through the SCR such that the maximum temperature and RMS current limits are not exceeded.

• The forward voltage drop across the SCR is very low when it is in conduction mode.

• The power loss in the thyristor is entirely depends on the forward current I_TAV.

RMS ON-state Current I

• This is the maximum repetitive RMS current specified at a maximum junction temperature that can flow through the SCR.

• For a direct current, both RMS and average currents are same.

• This rating is important for SCRs subject to low duty waveforms with peak currents.

• This rating is required to prevent excessive

heating in leads, metallic joints and interfaces of SCR.

Surge Current Rating I

• It specifies the maximum non-repetitive or

surge current that the SCR can withstand for a limited number of times during its life span.

I

Rating

• This rating is used to determine the thermal energy absorption of the device.

• This rating is required in the choice of a fuse or other protective equipment employed for the SCR.

• This is the measure of the thermal energy that the SCR can absorb for a short period of time before clearing the fault by the fuse.

di/dt Rating

• It is the maximum allowable rate of rise of anode to cathode current without any damage or harm to an SCR.

• If the rate of rise of anode current is very rapid compared to the spreading velocity of the charge carriers, local hot spots are created due to

concentration of carriers (on account of high current density) in the restricted area of the junctions.

• This raises the junction temperature above the safe limit and hence the SCR may be damaged.

Latching Current I

• It is the minimum ON state current required to maintain the SCR in ON state after gate drive has been removed.

• After turning ON of the SCR, the anode

current must be allowed to build up such that the latching current is attained before the gate pulse is removed.

• Otherwise the SCR will be turned OFF if the gate signal is removed.

Holding Current I

• This is the minimum value of the anode

current below which SCR stops conducting and turns OFF. The holding current is

associated with turn OFF process and usually it is a very small value in the range of mill

amperes.

Gate Current I

• As the gate current is more, earlier will be the turn ON of the SCR and vice-versa.

• However, safety limits must be provided for gate by specifying maximum and minimum gate currents.

• For controlling the SCR, gate current is applied to the gate terminal.

• This gate current is divided into two types; minimum gate current I_Gmin and maximum gate current I_Gmax.

• The minimum gate current I_Gmin is the current required by the gate terminal to turn ON the SCR where as

I_Gmax is the maximum current that can be applied safely to the gate.

Temperature Rating of SCR

• The forward and reverse blocking capability of

the SCR is determined by junction temperature T_j.

• If the maximum junction temperature is

exceeded, the SCR will be driven to conduction state even without any gate signal.

• This upper limit of T_j is imposed by considering the temperature dependence on break over

voltage, thermal stability and turn OFF time.

Power Ratings of SCR

• The power dissipation in the SCR produces a temperature rise in the junction regions.

• The dissipation of power in the SCR includes forward power dissipation; turn ON and OFF losses and gate power dissipation.

Average Power Dissipation P

• It is the multiplication of the average anode current and forward voltage drop across the SCR.

• This is the major source of junction heating in an SCR for normal duty cycle operations.

• The peak power from a given source must not exceed the average power dissipation rating to maintain the safety of the device.

Gate Power Dissipation P

• This rating defines both forward or reverse

peak power and the average power applied to the gate.

• If these ratings are exceeded, considerable damage occurs to the gate.

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